1. Field of the Invention
This invention relates to punch holders and more particularly to a punch holder-stripper assembly.
2. Prior Art
Metal punching devices, particularly sheet metal punching machines utilize a punch which is carried in a punch holder. The punch holder is larger than the punch and has an end face from which the punch projects. Although the specific shape of the punch holder differs from application to application, and may for example, differ for hard punch and die setups to be used in bed presses, punch-die units used in such presses or brake presses and drop-in holders used in single station and turret punch presses, a common feature is the fact that the holder has a larger dimension than the punch and has a bore into which the punch is received or through which the punch extends, the punch projecting beyond the punch holder and terminating in a punching end or tip.
Heretofore, it has been known to use stripping devices in association with punches to insure that the punched material is stripped from the punch tip. Such stripping devices have either been separate from the punch-punch holder unit or have been attached to the punch. A presently popular stripper construction consists of a polyurethane tube which is slipped over the projecting portion of the punch and grips the outer surface of the punch in a press fit or a stress fit relation.
In order to increase the ability of such strippers to be used with a wide variety of punches, it is common to form most punches with a cylindrical body section which thereafter tapers down to the tip, the tip being formed in whatever cross-section shape is desired.
Modifications of the elastomeric strippers heretofore used include simple tubes having a length sufficient to terminate a tube end adjacent the punch tip, closed end cup shaped units such as shown in the Archbold et al U.S. Pat. No. 3,234,835 and capped tube strippers such as shown in the Achler et al U.S. Pat. No. 3,485,122.
Although such prior art elastomeric strippers have represented a substantial advance in the art from prior used coil spring strippers, their method of attachment to the punch has numerous disadvantages. First, the inner diameter of the tube must be matched to the outer diameter of the punch body. Thus since a very wide range of punch body sizes exist, it has been necessary to stock a very large range of stripper sizes. Secondarily, numerous punches do not have a projecting portion which is cylindrical. Thus, the inside dimension and shape of the stripper had to be formed to accommodate the noncylindrical punch. This again required stocking of an additionally large number of stripper configurations and sizes. Thirdly, different punches for different applications project by differing amounts from the punch holder. In those instances where the amount of projection was relatively small, only very short stripper sections could be used. This can adversely effect the stripping force that can be generated, particularly when very small diameter axially short strippers were utilized. Fourthly, particularly where drop-in punch holders were utilized, such as in turret punch machines, the stripper always had to have a reduced outer diameter in order to allow clearance when inserted. Since many standard prior punch holders had relatively a small outer diameter dimension beyond the dimension of the punch, considerably less room was available for the stripper. Finally, since the strippers were received around the punch body and snugly engaged the punch body during compression of the stripper by movement of the punch through the workpiece, two undesirable problems occurred. First the stripper was compressed into tighter engagement with the punch body. Since there had to be relative movement between the punch body and the stripper, this compression created a friction which had to be overcome. Secondarily, a great deal of wear occurred on the inner dimension faces of the stripper.
It would therefore be an advance in the art to provide a stripper-punch holder combination which avoided a number of the above described deficiencies.